1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a technique of controlling overvoltage generated in an inductive load when forcedly stopping a switching device especially for the sake of protecting the semiconductor device.
2. Description of the Background Art
An ignition system for an internal combustion engine such as an automobile engine has a problem in that a control signal is held “on” due to a misoperation or the like during maintenance, causing a switching device for controlling current in an inductive load (e.g., transformer or load coil) to be kept conducting, which results in degradation in characteristics or breakdown of a semiconductor device itself or load due to heat generation.
Provided to solve this problem is a function of forcedly stopping a switching device using a timer circuit which operates after a lapse of a predetermined time period (approximately several hundred milliseconds) from the start of an ON operation. In other words, it is possible to avoid breakdown of the switching device by means of the timer circuit in the case where an ON signal stays output over a predetermined time period after the start of an ON operation due to a misoperation or the like.
In this case, a forced current interruption in the switching device may generate a great induced electromotive force in an inductive load, causing arc discharge to take place in an ignition coil with timing that a computer for controlling the ignition of an internal combustion engine is not intended to follow.
Accordingly, a ignition system limits a current interruption speed of a switching device to no more than a certain speed to control the degree of an induced electromotive force generated in an inductive load (cf. Japanese Patent Application Laid-Open No. 2002-4991).
Generally, however, output current is not linearly proportional to the gate voltage in a switching device. For instance, in an MOS gate device, output current is proportional to the square of the gate voltage. Therefore, a complicated circuit configuration and adjustment is required in order to control the switching device such that the current interruption speed is limited to no more than a certain speed.
For instance, the invention disclosed by the above-mentioned JP 2002-4991 controls an interruption speed of output current by means of the capacitance charging time. Since a large-capacitance capacitor is required to reduce the interruption speed of output current, it is difficult to reduce the circuit area. Further, an induced electromotive force varies depending on the inductance of an inductive load, which makes it necessary to vary the capacitance of the capacitor depending on the inductance of the inductive load. Therefore, it is difficult to control the induced electromotive force to be a desired clamping voltage without depending on the inductance of the inductive load.
Furthermore, when the potential at a current input terminal rises due to some unusual event while switching device is conducting, output current may increase, which causes damage to the switching device.